1. Field of the Invention
This invention relates to an ultrasonic ranging system and method.
2. Description of the Prior Art
Ultrasonic ranging systems are conventionally used in cameras to effect automatic focusing, in instruments for monitoring liquid level in containers, for proximity detectors, etc. U.S. Pat. No. 4,199,246, the disclosure of which is hereby incorporated by reference, is an example of an ultrasonic ranging system incorporated into a camera. The system in this patent transmits a burst of sonic energy towards a subject, and the transit time measured from the burst to receipt of an echo is a measure of subject range based on the assumption that the speed of sound is a constant. At room temperature, the speed of sound is about 340 msec; so that the time required for an ultrasonic burst to travel from a transducer to a subject 12 inches therefrom and then back to the transducer is about 1.78 msec.
In the patent referred to above, the burst transmitted by a transducer is in the form of a chirp whose frequency at the leading edge is about 65 kHz, and whose frequency drops to about 50 kHz within about 0.5 msec, and then remains constant for another 0.5 msec. The resultant burst thus lasts about 1 msec; and the provision of a variable frequency portion maximizes the possibility of obtaining a detectable return from subjects in the range of 3-8 feet, while the constant frequency portion is most effective for subjects more than 8 feet from the camera. As explained in the patent, a receiver for processing the output of the transducer is gain and bandwidth controlled and produces a range signal whose duration is proportional to subject range.
Inasmuch as a single transducer is used to both transmit the sonic energy and to receive echoes, proper operation requires the receiver to be blanked both during transmission of the burst and for a predetermined time thereafter. The latter operation is necessary in order to permit ringing of the transducer to terminate since only after such termination can the output of the receiver the associated with an echo. The time period that the receiver must be blanked depends on such parameters as the duration and power of the burst, etc; and for a 1 msec burst, about 600 usec following the burst is required for the ringing to subside significantly and permit the receiver to begin operation. Thus, for a 1 msec burst, the receiver is disabled for about 1.6 msec, with the result that a subject no closer than about 10 inches from the transducer can be detected. While this minimum subject distance is adequate for most camera work, in some instances and in other applications of ultrasonic ranging, detections of subjects closer than 10 inches is desirable.
When an ultrasonic ranging system of the type disclosed in the patent referred to above is used in environments other than cameras for distance measuring purposes, the usual procedure is to provide for a repetitious transmission of ultrasonic bursts and a memory for storing a representation of subject range, the memory being updated after each transmission occurs. In this general application of ultrasonic ranging, it is often necessary or desirable to detect subjects as far away as 30 feet. In order to ensure detection with this dynamic range, the transmitted ultrasonic burst should continue for a duration of about 1 msec. Inherently, this arrangement will not permit detection of subjects closer than about 10 inches.
It is, therefore, an object of the present invention to provide a new and improved ultrasonic ranging system wherein the problems and deficiencies outlined above are reduced or substantially overcome.